Approximately 70% of all gene therapy protocols are aimed at treating metastatic cancer. The majority of active protocols involve some form of cancer immunotherapy via cell-based gene transfer of cytokines or tumor antigens, while others involve the intratumoral delivery of oncolytic viruses or vectors bearing prodrugs, chemoprotective agents, antisense constructs, or tumor suppressor genes. However, the major unresolved problem that has hindered the development and deployment of effective cancer gene therapy is that of inefficient delivery to target cells in vivo, a problem that obviates and precludes many direct therapeutic approaches (Tseng and Mulligan, Surg. Oncol. Clin. N. Am. 11:537-569, 2002). In this regard, the advent of pathotropic targeting launches a new paradigm in cancer gene therapy. By targeting the histopathology of the lesion—rather than the cancer cells per se—to optimize the effective vector concentration at metastatic sites, the safety and the efficacy of the circulating gene therapy vector was increased dramatically in preclinical studies (Gordon et al., Cancer Res. 60:3343-3347, 2000; Gordon et al., Hum. Gene Ther. 12:193-204, 2001). Further enhanced by the inherent properties of the murine leukemia virus-based vector (which selectively transduces dividing cells) and the strategic specificity of a cell cycle control gene which exhibits tumoricidal and anti-angiogenic activities (Gordon et al., Hum. Gene Ther. 12:193-204, 2001), the preclinical and clinical performance of the pathotropic vector establishes the potential for systemic delivery of genetic medicine for the physiologic surveillance and treatment of primary, remote, metastatic, and occult cancers.
Improved vectors, systems for producing the improved vectors, and treatment regimens for administering such vectors, are desired so that targeted delivery systems can be employed in a clinical setting.